If transfers are not included for your trip, you can get shared-van service from the airport in Montego Bay to your final destination. Port Antonio is difficult to reach from MoBay at this writing, so Kingston is a better airport for that destination.
Renting a Car: The average traveler should not rent a car in Jamaica. Rentals are possible but are difficult to arrange on arrival, so make a reservation before your trip. You must be at least 23 years old, have a valid driver's license (from any country), and have a valid credit card. You may be required to post a security of several hundred dollars before taking possession of your car; ask about it when you make the reservation. Rates are also quite expensive, averaging $70 to $120 a day after the addition of the compulsory insurance, which you must usually purchase even if your credit card offers it.